The goal of this research is to implement a novel behavioral program designed to increase physical activity and enhance bone mineralization. The study focuses on sedentary adolescent females, a population at particularly high risk for developing osteoporosis later in life. The investigators hypothesize that the intervention will increase physical activity, that these increases in activity will be associated with changes in specific psychosocial characteristics, and that they will lead to increases in bone formation and bone mineral density over 10 months. The proposed research intends to: (1) test the intervention designed to increase female adolescents' physical activity; (2) evaluate the efficacy of one component of the intervention consisting of an Internet-based exercise diary linked with motivational role model stories; (3) evaluate the respective roles of specific psychosocial characteristics that may predispose toward the adoption and maintenance of physical activity; (4) determine whether positive changes in psychosocial characteristics over time are associated with increased participation in physical activity; and (5) test the hypothesis that increased physical activity enhances bone mineralization. Sedentary adolescent females will be assigned to full intervention (n=100), modified intervention (n=100), or comparison group (n=100) for 10 months. All participants will receive daily calcium supplementation. Bone formation will be assessed at baseline (DEXA), 5 wks and 5 mo. (serum and urinary measures), and 10 mo. (DEXA). Psychosocial characteristics and physical activity participation will be measure at all four intervals.